Figure 1: Both forms as releases of the same shape — only lib/ distinguishes them.

Two things.

The delivery. A directory cannot be copied like a file. What comes into being on the server comes into being from 128 files, and there is no way to replace 128 files in a single step.

The answer. The original title of this part was “Fat JAR or Thin Distribution?” — the question is answered only here, with measurements from the reference server. This much can be said in advance: one of the two metrics I had previously assumed to be equal is not.

A Promise from Part 1

Part 1, step 29 explicitly decided against the symlink for updates — releases are archived, the WAR is copied. Alongside that stood the sentence that the symlink approach would come later, “where it unfolds its purpose with the directory distribution.” Part 3, chapter 17 repeated it.

Chapter 4 redeems that promise — and explains why it would indeed have gained nothing for the WAR.

The Code State for This Part

The same as for Part 4: tagteil-04. The split into two articles concerns the text, not the repository —git diff teil-03 teil-04 shows both together.

git clone https://github.com/svenruppert/Blog-Vaadin-Deployment-on-Hetznercd Blog-Vaadin-Deployment-on-Hetznergit checkout teil-04

At tagteil-04, no bill of materials ships inside the release yet. The project does generate an SBOM, but it stays in the build directory. Why that is a deficiency and how it gets fixed is the subject of a separate article.

Building the Distribution with Maven

The components become a deliverable. For that, a second step joinscopy-dependencies from Part 4, chapter 8.

Assembly — the Right Tool Here

Themaven-assembly-plugin was eliminated as a route to the fat JAR in Part 4, chapter 3, because itsjar-with-dependencies descriptor does not merge service providers. For adirectory, this problem does not arise: nothing is merged, because nothing is flattened. Every archive remains an archive with its ownMETA-INF.

The descriptor lives atsrc/main/assembly/thin.xml:

<assembly><id>thin</id><formats><format>dir</format><format>tar.gz</format></formats><includeBaseDirectory>true</includeBaseDirectory><baseDirectory>vaadinapp-${project.version}</baseDirectory><fileSets><fileSet><directory>${project.basedir}/src/main/dist/bin</directory><outputDirectory>bin</outputDirectory><fileMode>0755</fileMode></fileSet></fileSets><files><file><source>${project.basedir}/src/main/dist/VERSION</source><outputDirectory>.</outputDirectory><filtered>true</filtered></file><file><source>${project.build.directory}/${project.build.finalName}.jar</source><outputDirectory>app</outputDirectory><destName>vaadinapp.jar</destName></file></files><dependencySets><dependencySet><outputDirectory>lib</outputDirectory><useProjectArtifact>false</useProjectArtifact><scope>runtime</scope></dependencySet></dependencySets></assembly>

Four places carry a decision.

dir andtar.gz at the same time. The directory afterwards sits intarget/ and can be transferred withrsync or inspected directly. The.tgz is the transport form for everything else. Both come from the same descriptor so they cannot drift apart.

tar.gz, notzip. A ZIP archive knows no execute permission in the POSIX sense. The start script would arrive without itsx bit, and the service would not start. For the case of long file names among the dependencies,<tarLongFileMode>posix</tarLongFileMode> is set in the plugin configuration — the defaultustar aborts on paths longer than 100 characters.

useProjectArtifact=false. Without this line, the application archive would end up inlib/, among the dependencies. Instead it is placed intoapp/ by hand — exactly the separation from Part 4, chapter 7.

filtered on theVERSION file. Maven substitutes${project.version} at build time. The same number thus appears in the archive and in the directory name without having to be maintained twice.

What Comes Out

$ mvn -Pproduction,thin package$ ls -l embedded-jetty/target/26676831 vaadinapp-00.01.00-thin.tar.gz10686 vaadinapp-embedded-jetty-00.01.00.jar vaadinapp-00.01.00-thin/$ find embedded-jetty/target/vaadinapp-00.01.00-thin -maxdepth2 -not -path'*/lib/*' vaadinapp-00.01.00-thin/vaadinapp-00.01.00 vaadinapp-00.01.00-thin/vaadinapp-00.01.00/VERSION vaadinapp-00.01.00-thin/vaadinapp-00.01.00/bin vaadinapp-00.01.00-thin/vaadinapp-00.01.00/app vaadinapp-00.01.00-thin/vaadinapp-00.01.00/lib$ ls -l …/bin/start.sh -rwxr-xr-x2608 start.sh

The execute permission is preserved. The proof that the delivery is complete is starting it from the unpacked directory:

[main] INFO Serving static web resources from 5 classpath root(s)
[main] INFO Started oejs.Server@17bffc17{STARTING}[12.1.12,sto=0] @1709ms
[main] INFO Embedded Jetty serving http://127.0.0.1:8099/

HTTP 200 on/ and/login, five resource roots, 1,709 milliseconds until the server is up. The last value is the telling one: it is in the same order of magnitude as the 1,523 ms from Part 3 and thus far above the signature of a scan that did not run.

The Same Shape for the Other Form

Thefatjar profile uses a second descriptor,fat.xml, which is identical except for two lines: it takes the shaded archive instead of the application archive, and it hasnodependencySet — there is nothing to copy intolib/, because everything already sits in the one file.

<files><file><source>${project.build.directory}/vaadinapp-${project.version}-fat.jar</source><outputDirectory>app</outputDirectory><destName>vaadinapp.jar</destName></file></files><!-- no dependencySet -->

ThefileSet forbin/ stays unchanged —the same start script, the same file. With that, both forms share the same shape:

vaadinapp-00.01.00/ vaadinapp-00.01.00/
├── VERSION ├── VERSION
├── bin/start.sh ├── bin/start.sh
├── app/vaadinapp.jar └── app/vaadinapp.jar
└── lib/ 127 archives

This is not an end in itself. Because the script putsapp/*:lib/* on the classpath and a missinglib/ resolves to nothing, the same file starts both forms. The systemd unit therefore does not distinguish them, and switching from one to the other is a symlink and a restart — which is what allows chapter 6 to measure them under equal conditions in the first place.

Two Profiles, One Module

Both packagings are profiles of the same module, for the reason from Part 4, chapter 1:

mvn -Pproduction,thin package# bin/, app/, lib/, and a .tgzmvn -Pproduction,fatjar package# bin/, app/, and a .tgz, without lib/mvn package# just the application archive

-Pproduction is required in both cases. Without this profile,build-frontend does not run, the production bundle is missing, and the bootstrap aborts with its own check — instead of offering a server that serves an empty page.

The third invocation without a profile is not an oversight. It produces an application archive for development in seconds, without copying 127 files or writing a 28 MB archive. Whoever builds the full deliverable on every development cycle gets used to waiting times nobody needs.

Delivering a Thin Distribution

Delivering a directory is more cumbersome than delivering a file. This chapter shows what the complications are and which of them can be resolved.

Restructuring the Target Directory

After Part 3, the reference server carries a flat layout:app.jar andlib/ directly under/opt/vaadinapp. Before releases can sit side by side, this has to become the structure from chapter 4:

sudo install -d -m750 -o root -g vaadinapp /opt/vaadinapp/releases

The existing state is not converted into a release. It comes from a delivery withoutbin/ and withoutVERSION and could not be started with the newExecStart. It stays in place as the last path of retreat until the first release runs, and is removed afterwards.

The Transfer

Two routes are available, and they differ in more than notation.

As an archive:

scp embedded-jetty/target/vaadinapp-00.01.00-thin.tar.gz\ sven@demo.svenruppert.com:/tmp/ssh sven@demo.svenruppert.com' STAMP=$(date -u +%Y-%m-%d-%H%M) sudo install -d -m 750 -o root -g vaadinapp /opt/vaadinapp/releases/$STAMP sudo tar -xzf /tmp/vaadinapp-00.01.00-thin.tar.gz \ -C /opt/vaadinapp/releases/$STAMP --strip-components=1 sudo chown -R root:vaadinapp /opt/vaadinapp/releases/$STAMP sudo find /opt/vaadinapp/releases/$STAMP -type f -exec chmod 640 {} + sudo chmod 750 /opt/vaadinapp/releases/$STAMP/bin/start.sh'

Withrsync:

rsync -a --delete\ embedded-jetty/target/vaadinapp-00.01.00-thin/vaadinapp-00.01.00/\ sven@demo.svenruppert.com:/tmp/release/

The archive transfers everything every time.rsync transfers what has changed — and for this form, that is the essential point, which chapter 6 quantifies in bytes. For thefirst release this makes no difference; for every subsequent one it does.

In return, the archive route has a propertyrsync does not: the transfer is one file and therefore atomic. What comes into being on the server comes into being only on unpacking, and that happens alongside the running operation. This combination — transfer an archive, operate a directory — is the third option from chapter 7.

Two Pitfalls Part 3 Learned the Hard Way

AppleDouble files. macOS’star creates companion files following the pattern._name for files with extended attributes and takes them along. Eleven of them ended up inlib/ in Part 3, and Java’s classpath wildcard picks up every file with the.jar extension — including these, which are not valid archives. The first start ended with HTTP 503.

COPYFILE_DISABLE=1 tar czf paket.tgz app lib bin

With delivery via themaven-assembly-plugin, this no longer arises: the archive is created inside the JVM from the files of the build directory, not via macOS’star. The pitfall concerns manual operation, and that does occur when individual files are updated by hand.

The permissions on the start script. Azip would lose the execute permission; achown -R followed by a blanketchmod would as well. That is why the commands above contain a dedicated line forbin/start.sh — and why chapter 2 builds the deliverable astar.gz with an explicit<fileMode>0755</fileMode>.

The Smoke Test

Before the symlink is repointed, the new release can be checked without touching the running operation:

sudo -u vaadinapp\JAVA_HOME=/var/lib/vaadinapp/.sdkman/candidates/java/current\JAVA_OPTS='-Dapp.port=8099 -Dapp.storage.dir=/tmp/probe'\ /opt/vaadinapp/releases/$STAMP/bin/start.sh

A second port, a different data directory, the same service user. Two indications from the log decide:

• Serving static web resources from 5 classpath root(s) — the resources are complete,
• a startup time in the range of seconds — the annotation scan took place.

A startup time in the double-digit millisecond range means the opposite, regardless of whether the server responds. This signature has now appeared twice: in Part 3 through an incomplete classpath, in Part 4, chapter 5 through a destroyed service provider directory.

Only once this check passes does the switch from chapter 4 follow.

Releases and thecurrent Symlink

Part 1 explicitly decided against the symlink for updates: releases are archived, the WAR is copied. Alongside that stood the sentence that the symlink approach would come in Part 4, “where it unfolds its purpose with the directory distribution.” This chapter redeems that promise — and begins with the question of why it wasnot worthwhile back then.

Why the Symlink Was Mere Decoration for the WAR

A WAR is a file.mv replaces a file atomically: either the old directory entry points to the old content or the new one to the new; there is no state in between. A symlink would merely have wrapped the same atomicity a second time.

A Thin Distribution consists of 128 files.There is no way to replace 128 files in a single step. Anrsync over the live tree leaves behind, for the duration of the transfer, a classpath in which some archives are new and others old. Whether that goes well depends on whether, during that window, a class is loaded that exists in both versions.

This is where the symlink earns its place: itrelocates the atomicity from the files to the name.ln -sfn rewrites a single directory entry — all or nothing.

The Directory Layout

/opt/vaadinapp/
├── current -> releases/2026-09-09-1200
├── releases/
│ ├── 2026-09-08-1730/{VERSION,bin,app,lib}
│ └── 2026-09-09-1200/{VERSION,bin,app,lib}
└── logs/

Timestamp instead of version number. The directories are named after the moment of delivery, not after the version. They therefore sort themselves and remain unambiguous when the same version is delivered twice — which happens regularly when a configuration is updated. Which version a release contains is stated in itsVERSION file.

logs/ and the data live outside. Logs and/var/lib/vaadinapp/data outlive the release switch. If the data directory sat undercurrent, a rollback would lose the data — a routine move would turn into an incident.

Permissions as with the WAR:root:vaadinapp, mode750. The service may read, not write. A compromised process can thus neither modify its current release nor write itself a next one.

The Switch

STAMP=$(date -u +%Y-%m-%d-%H%M)sudo install -d -m750 -o root -g vaadinapp /opt/vaadinapp/releases/$STAMPsudo tar -xzf /tmp/vaadinapp-00.01.00-thin.tar.gz\ -C /opt/vaadinapp/releases/$STAMP --strip-components=1sudo chown -R root:vaadinapp /opt/vaadinapp/releases/$STAMPsudo ln -sfn releases/$STAMP /opt/vaadinapp/currentsudo systemctl restart vaadinapp

Six lines, two of which constitute the actual switch. Everything before that happensalongside the running operation: the new release is laid down in full while the application keeps running from the old one.

ln -sfn with both switches is no ornament here. Without-n,ln follows the existing symlink and creates the new oneinside the target directory — the result isreleases/2026-09-08-1730/2026-09-09-1200, andcurrent still points to the old release. The command reports no error.

What Happens to the Running Process When the Symlink Is Repointed: Nothing

This is the property the whole procedure rests on, and it can be demonstrated. Two releases side by side,current points to the first, the application is running. Then the symlink is repointed,without a restart:

$ ls -l current
current -> releases/2026-09-09-1200
$ curl -o /dev/null -w '%{http_code}\n' http://127.0.0.1:8095/
200
$ ln -sfn releases/2026-09-09-1400 current
$ ls -l current
current -> releases/2026-09-09-1400
$ curl -o /dev/null -w '%{http_code}\n' http://127.0.0.1:8095/
200
$ lsof -p 71775 | grep -c 2026-09-09-1200
139
$ lsof -p 71775 | grep -c 2026-09-09-1400
0

The process keeps serving requests and holds139 open files in the old release, not a single one in the new. The reason lies below Java: a path is resolved on open, not on every access. Whatever happens to the name afterwards no longer concerns the open descriptor.

Two things follow from this at once:

• The switch needs a restart. Without one, the old version would keep running indefinitely while the symlink claims otherwise.
• The rollback is safe. The old release sits untouched on disk — not as an archive that would first have to be unpacked, but ready to start.

Cleanup

At roughly 30 MB per state,releases/ grows faster than one notices. The last five are enough:

ls -1d /opt/vaadinapp/releases/*/| sort| head -n -5|\ xargs -r sudo rm -rf

This line belongs at theend of the rollout, not at its beginning: first the new release is proven to run, then old ones are cleared away. The other way around, you would be left without a rollback target in case of failure.

Rollback

The rollback is the reason the procedure of the previous chapter is operated at all. It consists of two lines:

sudo ln -sfn releases/2026-09-08-1730 /opt/vaadinapp/currentsudo systemctl restart vaadinapp

This is the same motion as the switch, just with a different target. There is no separate procedure for the failure case, no unpacking from an archive, no restoring from a backup.

Measured on the reference server — from repointing the symlink to the first request answered again:4,938 milliseconds. That is the same order of magnitude as an ordinary restart (4,653 ms on average), because nothing else takes place. The duration does not depend on how large the application is or how far back the rollback reaches.

Why That Is Enough

Because the old release was never touched. Chapter 4 showed that a running process holds its files through open descriptors and notices nothing when the symlink is repointed — 139 open files in the old release, none in the new. The flip side of the same property is that the old release remainsready to start as long as it is not cleared away.

The time to recovery therefore does not depend on the size of the application, only on the duration of a restart. That is the real gain over the procedure from Part 1, where a WAR had to be copied back from the archive.

What the Rollback Does Not Take Back

Three things, and all three need to be considered beforehand.

The sessions. They live in memory, as they have since Part 2. A restart discards them, and logged-in users land on the login page — on a switch as on a rollback. This is not a property of this procedure but a consequence of how sessions are held, and it could only be avoided with persistent sessions.

The data./var/lib/vaadinapp/data lives outside the releases and is not reset. That is intentional: a rollback is meant to take back the application, not the users’ work. It does mean, however, that a release that changed the data — a migration — cannot be taken back this way. The symlink makes the code reversible, not the schema.

The unit. It lives under/etc/systemd/system and belongs to the machine. This is exactly why the cut from Part 4, chapter 9 is placed where it is: because the release’s JVM options live inside the release, the rollback brings them along. If they lived in the unit, someone would have to remember to take them back as well — and would forget in the heat of a failure.

The Procedure When It Gets Serious

# 1. Which releases are available?ls -1 /opt/vaadinapp/releases/# 2. What does current point to right now?readlink /opt/vaadinapp/current# 3. Which version is in the target?cat /opt/vaadinapp/releases/2026-09-08-1730/VERSION# 4. Switch back and restartsudo ln -sfn releases/2026-09-08-1730 /opt/vaadinapp/currentsudo systemctl restart vaadinapp# 5. Verify, do not assumesystemctl is-active vaadinappcurl -s -o /dev/null -w'%{http_code}\n' https://demo.svenruppert.com/journalctl -u vaadinapp -n20 --no-pager

Step 3 is the one that gets skipped, and the one that justifies theVERSION file from Part 4, chapter 7. The directory names are timestamps; which of them contains the version to roll back to, only this file can say.

Step 5 is the one that runs through the whole series: the symlink can point correctly and the service still fail to run, for instance because the old release has a start script that expects an environment variable removed in the meantime. A rollback is proven only when a request is answered.

The Measured Comparison

Unlike in Parts 2 and 3, it is not the server that changes here. Both forms load the same classes from the same archives with the same bootstrap. A table in which half the rows show identical numbers proves nothing — which is why there are five rows, and two of them have not appeared in this series before.

Measurements were taken on the reference server, as in all previous parts: Debian 13, Temurin 26, the same unit, the same hardening. Both forms sit side by side as releases; switching between them is a symlink and a restart — only that makes the numbers comparable.

The hypotheses were written downbefore the measurement. Two of them were not confirmed, and both times the opposite is the more instructive result.

Delivery Size

Both forms are delivered as releases and transferred as.tgz for that purpose — same structure, same transport form, so the numbers are comparable.

The hypothesis was: the fat JAR is somewhat smaller, because 127 archives each carry their own central directory.Confirmed — the fat form is smaller by 868,980 bytes, or 3.3 percent.

The third row deserves a warning, because it invites a fallacy. Compare theraw fat JAR with the.tgz of the Thin Distribution and the result reverses: 28.89 versus 26.68 MB. But that is not a comparison of two delivery forms; it is a comparison of once-compressed and twice-compressed data. A JAR compresses each entry on its own;gzip over thetar stream compresses once more on top and exploits repetitions across file boundaries. Whoever measures this way measures the packaging of the packaging.

The amount is small in both directions. For the choice between the forms, size yields nothing — it is listed here mainly because the obvious mismeasurement leads so easily to a false statement.

Startup Time

Three runs per form on the reference server, Jetty’s own timestamp up toStarted Server, plus the downtime fromrestart to the first answered request:

The hypothesis was: the fat JAR starts faster, because Jetty’sMetaInfConfiguration opens one archive instead of 127.Not confirmed. The difference is 92 milliseconds against a spread of 83 and 322 milliseconds, respectively, within the runs — the ranges overlap completely. The same measurement on the development machine yielded 1,654 versus 1,671 milliseconds, that is, the opposite direction at an equally small amount.

That is the more instructive finding. It sayswhere the time actually goes: not into opening the archives, but into the ASM pass over the class files inside them. Their number is the same in both forms. Creating 127 file handles carries no weight next to that.

This also puts the number from Part 3 into perspective. The 139 milliseconds versus 1,523 milliseconds there werenot the price of 127 opened archives, but the difference between a scan that happens and one that does not. Whoever derives from it the expectation that fewer archives would bring a faster start has misread the measurement — and this chapter is the cross-check on that.

Memory Footprint

This is where the most definite hypothesis stood, and it is the second one to fall.

The hypothesis was: no difference — same classes, same bootstrap.Refuted. The fat JAR needs48 MiB more, which is 13 percent. The values per form sit close together (358 to 362 versus 403 to 408); the ranges do not touch. That is not noise.

For context: Part 3 measured 336 MiB for the flat layout; the Thin Distribution as a release, at 358 MiB, is in the same order of magnitude.

Where the 48 MiB Come From — and Where They Do Not

The obvious explanation would be that a 28 MB archive is mapped into memory as a whole. A look at/proc/<pid>/maps refutes that:

 Thin Fat
file-backed mappings
on .jar files 0 0
mappings total 332 331
open archive descriptors 139 4

Not a single archive is memory-mapped — in neither form. The JVM reads the archives’ directories into ordinary memory; it does not map the files.

/proc/<pid>/smaps_rollup shows where the difference sits instead:

 Thin Fat
Rss 384 MiB 436 MiB
Private_Dirty 358 MiB 410 MiB ← the entire difference
Private_Clean 23 MiB 23 MiB
Shared_Clean 2 MiB 2 MiB

The increase sitsentirely in anonymous, dirty memory — heap or natively requested JVM memory, not in file mappings and not in the page cache.

This narrows the cause down, but does not determine it. What it is not is stated above; what it is, this measurement does not yield. The number was taken ten seconds after startup in each case, that is, before garbage collection has settled — a larger retained set immediately after the scan is conceivable and would have to be examined separately.

What the finding saysfor certain: the statement from Part 3 — that what is measured is not the number of bytes shipped but the number of loaded classes — falls short. Here the classes are the same, and the footprint differs anyway. The form of delivery is not without consequence for memory.

Change Volume for a Patch Release

The first of the two new rows — and the only one with a clear difference.

Of the 127 archives inlib/,126 are byte-identical to the files in the local Maven repository. They come from there unchanged and do not change between two builds. Only the project’s own archives are rewritten.

So if only the application code changes:

rsync transfers even less in both cases, because it matches blocks within changed files. That changes nothing about the ratio: with the fat JAR,every change is a change to the same 28 MB file.

A note on this application: the 2.59 MB is thecore module, which contains the entire application code and the Vaadin production bundle. In a project without this separation, the application archive would stand here alone — ten kilobytes. The ratio would then not be 11 to 1 but 2,700 to 1.

Diagnostic Effort

The second new row, without a number, but with a demonstrable difference.

# Thin Distribution$ ls /opt/vaadinapp/current/lib| grep bcprovbcprov-jdk18on-1.84.jar# Fat JAR$ unzip -l /opt/vaadinapp/current/app.jar| grep -i bouncycastle(nothing — the classes live under org/bouncycastle/, the version nowhere)

The difference is not that the second question is harder to answer. Itcannot be answered. The classes are there; the provenance is gone. The only substitute is the SBOM from build time — which for the fat JAR is thus not an accessory but the only remaining source.

What the Measurement Does Not Yield

Threads were collected (53 versus 49) and are not reported: there was no hypothesis stated in advance, and without one, a deviation between two runs is not a finding but noise.

Build time differs — Shade writes a 28 MB archive,copy-dependencies copies files. It is a property of the development machine, not of the delivery, and this series measures at the server.

Summary

Size goes narrowly to the fat JAR, startup time to no one, diagnostic effort and the patch release clearly to the Thin Distribution. None of these rows decides the choice on its own — and the fat JAR’s real strength appears in none of them, because it cannot be captured in a number. Chapter 7 draws the conclusion.

Recommendation

Both forms run. Both deliver HTTP 200. Startup time does not differ; delivery size differs by three percent in the fat JAR’s favor. Up to that point, a measurement would yield little for a recommendation.

Two rows from chapter 6 do after all, and both point in the same direction: the fat JAR needs48 MiB more memory (406 versus 358 MiB), and a patch release transferseleven times as many bytes with it. Add to that what cannot be measured but affects every day of operation: with the Thin Distribution, the question “which version is deployed?” is answered with anls; with the fat JAR, not at all.

The Recommendation Depends on the Operational Situation

The sentence it comes down to:

The Third Option

The juxtaposition “one archive or many files?” contains an assumption that does not hold: that the answer fortransferring and forrunning would have to be the same. It does not.

On the reference server, both forms are transferred as.tgz and unpacked into a release directory. The transfer is thusone file in both cases — atomic, verifiable with a checksum, without the danger of 128 files arriving halfway. What sits in the directory afterwards is independent of that.

Both releases share the same shape:

vaadinapp-00.01.00/ vaadinapp-00.01.00/
├── VERSION ├── VERSION
├── bin/start.sh ├── bin/start.sh ← same file
├── app/vaadinapp.jar └── app/vaadinapp.jar (28,89 MB)
└── lib/ 127 archives

The start script isbyte-for-byte identical. It puts"$APP_HOME/app/*:$APP_HOME/lib/*" on the classpath; iflib/ is missing, the second half resolves to nothing, and the one complete archive inapp/ is all the annotation scan needs — exactly the property from Part 4, chapter 4, that the fat JAR rests on.

The consequence is practical: the systemd unit does not distinguish the two forms. Switching from one to the other is a symlink and a restart, that is, the same operation as any release switch. That is the reason the two forms could be measured under equal conditions in chapter 6 at all.

What this third option isnot: a size advantage. As.tgz, both sit at roughly 25 to 26 MB, and the fat form is even the slightly smaller one. The gain lies solely in the fact that the question of atomicity in transfer and the question of visibility in operation can be answered separately.

What Does Not Work as a Justification Here

“Modern” or “best practice.” The fat JAR is, thanks to Spring Boot, the more common route. That does not make it the better one — it makes it the expected one, and that is an argument about habits, not about technology.

“The proper Unix way.” The opposite direction is just as unfit.bin/,lib/, and a start script are an old and proven form, but their lineage proves nothing about their suitability for this case.

Both forms are old, both are in use, and the choice follows from the situation, not from fashion.

What Runs on the Reference Server

The Thin Distribution, transferred as.tgz and unpacked into a release directory — that is, the third route. This is not hedging in all directions, but the consequence of the same reasoning: the machine is managed in-house, so visibility into its components counts. And the transfer should still be atomic.

The state after this part:

/opt/vaadinapp/
├── current -> releases/2026-09-09-thin
├── logs/
└── releases/ seven releases, from the WAR archives of part 1
up to both forms of this part

Both forms remain in place as releases. Switching between them is thus, at any time, a symlink and a restart — measured: 4,938 milliseconds.

That Part 6 builds on this is a bonus, but decides nothing. A recommendation that follows from the next part instead of from the matter at hand would be none.

Outlook on Part 6

Parts 4 and 5 have not moved any boundary. They packaged and delivered what Part 3 built, and in doing so answered two questions that had been open since then: what the delivery looks like, and how a release is switched and taken back.

What the application still expects from the target system has thereby melted down to a single thing: a Java runtime, installed and maintained by someone else. On the reference server that is a Temurin 26, set up via SDKMAN under the service user, reachable through a symlink — the same mechanism Part 1 used for Jetty, and which has since been dropped there.

Part 6 takes the runtime along as well.jlink builds, from the modules of the JDK, an image that contains only what this application calls, and places it next to the distribution.ExecStart=…/current/bin/start.sh remains the same line — onlyJAVA_HOME then points into the release instead of at the machine. The server no longer needs an installed Java after that.

Two things need to be said up front so they do not appear as surprises.jlink does not make the application modular: it builds an image of the platform modules; the application code stays on the classpath. The recommendation from chapter 7 is untouched by this — whoever chose the fat JAR gets ahead just the same. Andjdeps will not deliver the list of required modules completely: Vaadin loads via reflection, Jetty viaServiceLoader, and static analysis does not see that.

This is the same finding that holds this part and the previous one together, for the third time:what is decided at build time about runtime must go beyond what the compiler sees. In Part 3 it was the classpath that fell short. In Part 4, chapter 5 it was the service provider directory that a merge without knowledge of its contents would have destroyed. In Part 6 it will be the module list.